Organic photovoltaics (OPVs) offers key advantages such as flexibility and chemical tunability, however, is struggling due to the lower power conversion efficiency compared to other photovoltaic systems. A donor-acceptor conjugated block copolymer is one of the most promising approaches in order to adopt a periodic nanostructure in the length scale commensurate with the exciton diffusion length. Also, it can serve as the simplest model system, comprised of one donor and one acceptor linked by a covalent bond, to study intra-molecular energy transfer. We investigated the energy transfer mechanism by manipulating the distance between the donor and acceptor blocks of conjugated block copolymers. Conjugated block copolymers have two possible routes for energy transfer: through-bond (i.e., through the covalent bond between the donor and acceptor blocks) and through-space. Through-space energy transfer is governed by the Forster resonance energy transfer (FRET) which is a highly sensitive process to the distance between the donor and acceptor blocks. We used a conjugated block copolymer comprised of poly(3-hexylthiophene-2,5-diyl) (P3HT) as the donor and poly((9,9-bis-octylfluorene-2,7-diyl)-<i>alt</i>-(4,7-di(thiophene-2-yl)-2,1,3-benzothiadiazole)-5′,5″-diyl) (PFTBT) as the acceptor. To increase the distance between the donor and the acceptor, an additional fluorene unit was introduced as a bridging unit. Time resolved fluorescence anisotropy (TRFA) was examined for the conjugated block copolymer with and without a bridging unit. Interestingly, in the case of without a bridging unit, the anisotropy decay was faster that indicates efficient FRET between donor and acceptor. Therefore, it is confirmed that intra-molecular energy transfer can occur by ‘through-space’ and it is very sensitive on the distance in the length scale of one repeating unit of a conjugated block copolymer.

Understanding the energy transfer mechanism of donor-acceptor conjugated block copolymers for organic photovoltaics

ACS Fall 2020 Virtual Meeting

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